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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

To: "Tom Cathey" <>, <>
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "kenw2dtc" <>
Date: Wed, 14 Dec 2005 14:07:41 -0500
List-post: <>

Gates had a commercial amplifier (MODEL ST-3A) with 2nd harmonics down 50 db 
and all higher order down at least 60 db.  The input was 100 mw and the tube 
line up was: a 6CL6 in class A, an 8122 in class A and a 4CX3000A in AB1. 
The output was rated at 3000 watts.


Ken Barber  Middletown, NJ
Radio Website:

----- Original Message ----- 
From: "Tom Cathey" <>
To: <>
Sent: Wednesday, December 14, 2005 12:49 PM
Subject: [Amps] Designing the Cleanest Linear with RF Negative Feedback

> Hi All,
> I'm planning to build up a 75M monoband amplifier that has at least -45db
> to -50db 3rd order IMD using heavy RF negative feedback. 10MW in, 1500W 
> out.
> I wanted to get opinions here what others thought was the best general
> configuration and perhaps suggest some real world tubes.
> It seems that the best amps that use RF feedback start with a much larger
> tube than generally used in open loop linears and run it about half power 
> .
> I plan to use the 10 milliwatt output from my FT-1000D as a source. This 
> is
> the low level stage just before it goes into the power amp board. I'm 
> hoping
> this is class A stuff and a  much cleaner starting point than its -32db 
> 200W solid state power amplifier board output. Any opinions on that?
> For this amplifier I am thinking of either a chain of grounded grid stages
> so that they have individual built in feedback and an additional loop of
> feedback around the whole chain.
> OR, use a tetrode final with regulated screen voltage with stages going 
> back
> to amplify the 10MW level. Hopefully, use up to 15-20db of  loop feedback
> assuming the gain is available.
> I would appreciate any pointers to URLs showing working schematics of the
> big commerical amps that Collins, Harris, etc use, and ideas you may have
> for a design.
> For example... I don't know what good low level tubes to use, but lets 
> just
> say it went something like,  6AU6, 12BY7, 6146, 811A, 3-500Z,  GS-35B  - 
> all
> in grounded grid with an additional feedback loop.
> Or, for grid driven:  The same low level stages, then 4CX-150, 4-1000A,
> 4CX-3000.  Looking for conservative ratings at 1500W and high IMD figures.
> Another thing I wonder about is if a tube like the 6146 or 4-1000A which 
> has
> modest IMD figures open looped,  can successfully participate in a -45
> to -50db IMD chain when big feedback is used?
> I will be building this amp over the next few months and will post the
> results once completed. Thanks in advance for any help!
> 73,
> Tom, K1JJ
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